doxygen layout file with missing elements: is it valid?

So I have a doxygen layout XML file that I'm using for the sole
purpose of just adding a tab group with some tabs under it. I don't
care about the rest of it. Most importantly, when I upgrade Doxygen in
the future, I want the rest of the stuff to generate default like it
normally does as if the layout XML wasn't specified explicitly. I ran
a test with the following layout XML file contents:

<doxygenlayout version="1.0">
  <!-- Generated by doxygen 1.8.13 -->
  <!-- Navigation index tabs for HTML output -->
  <navindex>
    ...
  </navindex>
</doxygenlayout>

Obviously I have <tab> elements under <navindex>; assume all of that
is valid. Notice I removed every other top-level element under
<doxygenlayout>. Seems to work fine (browsed a class's documentation).
Didn't notice any visual issues.

Is this valid? Will doxygen use internal defaults for missing
top-level elements in the layout XML?

------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot